The invention relates to fibers for reinforcing matrix materials, and more particularly to a plurality of synthetic polymer fibers having excellent dispersibility and reinforcibility properties in hydratable cementitious compositions. Individual fiber bodies are elongated and highly bendable, with generally quadrilateral cross-sectional profiles, thereby minimizing fiber balling and maximizing fiber bond.
Although fibers of the present invention are suitable for reinforcing various matrix materials, such as adhesives, asphalts, composites, plastics, rubbers, etc., and structures made from these, the fibers that will be described herein are especially suited for reinforcing hydratable cementitious compositions, such as ready-mix concrete, precast concrete, masonry concrete (mortar), shotcrete, bituminous concrete, gypsum compositions, gypsum- and/or Portland cement-based fireproofing compositions, and others.
A major purpose of the fibers of the present invention is to reinforce concrete, e.g., ready-mix, shotcrete, etc., and structures made from these. Such matrix materials pose numerous challenges for those who design reinforcing fibers.
Concrete is made using a hydratable cement binder, a fine aggregate (e.g., sand), and a coarse aggregate (e.g., small stones, gravel). A mortar is made using cement binder and fine aggregate. Concretes and mortars are hence brittle materials. If a mortar or concrete structure is subjected to stresses that exceed its maximum tensile strength, then cracks can be initiated and propagated therein. The ability of the cementitious structure to resist crack initiation and crack propagation can be understood with reference to the xe2x80x9cstrengthxe2x80x9d and xe2x80x9cfracture toughnessxe2x80x9d of the material.
xe2x80x9cStrengthxe2x80x9d relates to the ability of a cement or concrete structure to resist crack initiation. In other words, strength is proportional to the maximum load sustainable by the structure without cracking and is a measure of the minimum load or stress (e.g., the xe2x80x9ccritical stress intensity factorxe2x80x9d) required to initiate cracking in that structure.
On the other hand, xe2x80x9cfracture toughnessxe2x80x9d relates to the specific xe2x80x9cfracture energyxe2x80x9d of a cement or concrete structure. This concept refers to the ability of the structure to resist propagationxe2x80x94or wideningxe2x80x94of an existing crack in the structure. This toughness property is proportional to the energy required to propagate or widen the crack (or cracks). This property can be determined by simultaneously measuring the load required to deform or xe2x80x9cdeflectxe2x80x9d a fiber-reinforced concrete (FRC) beam specimen at an opened crack and the amount or extent of deflection. The fracture toughness is therefore determined by dividing the area under a load deflection curve (generated from plotting the load against deflection of the FRC specimen) by its cross-sectional area.
In the cement and concrete arts, fibers have been designed to increase the strength and fracture toughness in reinforcing materials. Numerous fiber materials have been used for these purposes, such as steel, synthetic polymers (e.g., polyolefins), carbon, nylon, aramid, and glass. The use of steel fibers for reinforcing concrete structures remains popular due to the inherent strength of the metal. However, one of the concerns in steel fiber product design is to increase fiber xe2x80x9cpull outxe2x80x9d resistance because this increases the ability of the fiber to defeat crack propagation. In this connection, U.S. Pat. No. 3,953,953 of Marsden disclosed fibers having xe2x80x9cJxe2x80x9d-shaped ends for resisting pull-out from concrete. However, stiff fibers having physical deformities may cause entanglement problems that render the fibers difficult to handle and to disperse uniformly within a wet concrete mix. More recent designs, involving the use of xe2x80x9ccrimpedxe2x80x9d or xe2x80x9cwave-likexe2x80x9d polymer fibers, may have similar complications, depending on the stiffness of the fiber material employed.
Polyolefin materials, such as polypropylene and polyethylene, have been used for reinforcing concrete and offer an economic advantage due to relative lower cost of the material. However, these polyolefinic materials, being hydrophobic in nature, resist the aqueous environment of wet concrete. Moreover, the higher amount of polyolefin fibers needed in concrete to approximate the strength and fracture toughness of steel fiber-reinforced concrete often leads to fiber clumping or xe2x80x9cballingxe2x80x9d and increased mixing time at the job site. This tendency to form fiber balls means that the desired fiber dosage is not achieved. The correct concentration of fibers is often not attained because the fiber balls are removed (when seen at the concrete surface) by workers intent on achieving a finished concrete surface. It is sometimes the case that locations within the cementitious structure are devoid of the reinforcing fibers entirely. The desired homogeneous fiber dispersion, consequently, is not obtained.
Methods for facilitating dispersion of fibers in concrete are known. For example, U.S. Pat. No. 4,961,790 of Smith et al. disclosed the use of a water-soluble bag for introducing a large number of fibers into a wet mix. U.S. Pat. No. 5,224,774 of Valle et al. disclosed the use of non-water-soluble packaging that mechanically disintegrated upon mixing to avoid clumping and to achieve uniform dispersal of fibers within the concrete mix.
The dispersal of reinforcing fibers could also be achieved through packaging of smaller discrete amounts of fibers. For example, U.S. Pat. No. 5,807,458 of Sanders disclosed fibers that were bundled using a circumferential perimeter wrap. According to this patent, the continuity of the peripheral wrapping could be disrupted by agitation within the wet concrete mix, and the fibers could be released and dispersed in the mix.
On the other hand, World Patent Application No. WO 00/49211 of Leon (published Aug. 24, 2000) disclosed fibers xe2x80x9cpacketedxe2x80x9d together but separable when mixed in concrete. A plurality of fibers were separably-bound together, such as by tape adhered to cut ends of the fibers, thereby forming a xe2x80x9cpacket.xe2x80x9d Within a wet cementitious mix, the packets could be made to break and/or dissolve apart to permit separation and dispersal of individual fibers within the mix.
The dispersal of reinforcing fibers may also be achieved by altering fibers during mixing. For example, U.S. Pat. No. 5,993,537 of Trottier et al. disclosed fibers that progressively fibrillated upon agitation of the wet concrete mix. The fibers presented a xe2x80x9clow initial surface areaxe2x80x9d to facilitate introducing fibers into the wet mix, and, upon agitation and under the grinding effect of aggregates in the mix, underwent xe2x80x9cfibrillation,xe2x80x9d which is the separation of the initial low-surface-area fibrous material into smaller, individual fibrils. It was believed that homogeneous fiber distribution, at higher addition rates, could thereby be attained.
A novel approach was taught in U.S. Pat. No. 6,197,423 of Rieder et al., which disclosed mechanically-flattened fibers. For improved keying within concrete, fibers were flattened between opposed rollers to attain variable width and/or thickness dimensions and stress-fractures perceivable through microscope as discontinuities and irregular and random displacements of polymer on the surface of the individual fibers. This microscopic stress fracturing was believed to improve bonding between cement and fibers, and, because the stress-fractures were noncontinuous in nature, the fibers were softened to the point at which fiber-to-fiber entanglement (and hence fiber balling) was minimized or avoided. The mechanical-flattening method of Rieder et al. was different from the method disclosed in U.S. Pat. No. 5,298,071 of Vondran, wherein fibers were interground with cement clinker and retained cement particles embedded into the surface.
In this vein, the nature of the fiber surface has also been a frequent topic of research in fiber dispersion and bonding in concrete. For example, U.S. Pat. No. 5,753,368 of Hansen disclosed a list of wetting agents such as emulsifiers, detergents, and surfactants to render fiber surfaces more hydrophilic and thus more susceptible to mixing in wet concrete. On the other hand, U.S. Pat. No. 5,753,368 of Berke et al. taught that the bonding between concrete and fibers could be enhanced by employing particular glycol ether coatings instead of conventional wetting agents that tended to introduce unwanted air at the fiber/concrete interface.
Of course, as mentioned in U.S. Pat. No. 5,298,071 and U.S. Pat. No. 6,197,423 as discussed above, physical deformation of the fiber surface was also believed to improve the fiber-concrete bond. U.S. Pat. No. 4,297,414 of Matsumoto, as another example, taught the use of protrusions and ridges to enhance bond strength. Other surface treatments, such as the use of embossing wheels to impose patterns on the fiber, were also used for improving fiber-concrete bond. Fiber designers have even bent fibers into sinusoidal wave shapes to increase the ability of fibers to resist being pulled out from concrete. However, the present inventors realized that increased structural deformations in the fiber structure may actually enhance opportunities for unwanted fiber balling to occur.
Against this background, the present inventors see a need for novel polymeric synthetic reinforcing fibers having ease of dispersibility in concrete so as to avoid fiber balling and to achieve intended fiber dosage rates, while at the same time to provide strength and fracture toughness in matrix materials and particularly brittle materials such as concrete, mortar, shotcrete, gypsum fireproofing, and the like.
In surmounting the disadvantages of the prior art, the present invention provides highly dispersible reinforcing polymer fibers, matrix materials reinforced by the fibers, and methods for obtaining these. Exemplary fibers of the invention provide ease of dispersibility into, as well as strength and fracture toughness when dispersed within, matrix materials, particularly brittle ones such as concrete, mortar, gypsum or Portland cement-based fireproofing, shotcrete, and the like.
These qualities are achieved by employing a plurality of individual fiber bodies having an elongated length defined between two opposing ends, the bodies having a generally quadrilateral cross-sectional shape along the elongated length of the fiber body. The individual fibers thereby have a width, thickness, and length dimensions wherein average width is 1.0-5.0 mm and more preferably 1.3-2.5 mm, average thickness is 0.1-0.3 mm and more preferably 0.15-0.25 mm., and average length is 20-100 mm and more preferable 30-60 mm. In preferred embodiments, average fiber width should exceed average fiber thickness by at least 4 times (i.e., a ratio of at least 4:1) but preferably average width should not exceed average thickness by a factor exceeding 50 times (50:1). More preferably, the width to thickness ratio of the fibers is from 5 to 20 (5:1 to 20:1).
While individual fiber bodies of the invention may optionally be introduced into and dispersed within the matrix material as a plurality of separate pieces or separable pieces (ie. fibers in a scored or fibrillatable sheet, or contained within a dissolvable or disintegratable packaging, wrapping, packeting, or coating) the fibers can be introduced directly into a hydratable cementitious composition and mixed with relative ease to achieve a homogeneous dispersal therein. Individual fiber bodies themselves, however, should not be substantially fibrillatable (i.e. further reducible into smaller fiber units) after being subjected to mechanical agitation in the matrix composition to the extent necessary to achieve substantially uniform dispersal of the fibers therein.
Exemplary individual fiber bodies of the invention are also substantially free of internal and external stress fractures, such as might be created by clinker grinding or mechanical flattening. The general intent of the present inventors is to maintain integrity of the individual fiber bodies, not only in terms of structural fiber integrity, but also integrity and uniformity of total surface area and bendability characteristic from one batch to the next.
A generally quadrilateral cross-sectional profile provides a higher surface area to volume ratio (Sa/V) compared to round or oval monofilaments comprising similar material and having a diameter of comparable dimension. The present inventors believe that a quadrilateral cross-sectional shape provides a better flexibility-to-volume ratio in comparison with round or elliptical cross-sectional shapes, and, more significantly, this improved flexibility characteristic translates into better xe2x80x9cbendabilityxe2x80x9d control. The individual fiber bodies of the invention will tend to bend predominantly in a bow shape with comparatively less minimal twisting and fiber-to-fiber entanglement, thereby facilitating dispersal. In contrast, for a given material modulus and cross-sectional area, the prior art fibers having circular or elliptical cross section with major axis/minor axis ratios of less than 3 will have greater resistance to bending, thereby having a greater tendency for fiber balling when compared to fibers of generally quadrilateral (e.g., rectangular) cross-section.
The present inventors further believe that a generally quadrilateral cross-section will provide excellent fiber surface area and handability characteristics when compared, for example, to round or elliptical fibers. In this connection, preferred fibers of the invention have a xe2x80x9cbendabilityxe2x80x9d in the range of 20 (very stiff) to 1300 (very bendable) milli Newtonxe2x88x921*meterxe2x88x922 (mNxe2x88x921 mxe2x88x922), and more preferably in the range of 25 to 500 milli Newtonxe2x88x921*meterxe2x88x922. As used herein, the term xe2x80x9cbendabilityxe2x80x9d means and refers to the resistance of an individual fiber body to flexing movement (ie. to force that is perpendicular to the longitudinal axis of the fiber) as measured by applying a load to one end of the fiber and measuring its relative movement with respect to the opposite fiber end that has been secured, such as within a mechanical clamp or vice, to prevent movement. Thus, a fiber can be called more bendable if it requires less force to bend it to a certain degree. The bending flexibility of a fiber is a function of its length, shape, the size of its cross-section, and its modulus of elasticity. Accordingly, the bendability xe2x80x9cBxe2x80x9d of the fiber is expressed in terms of milli Newtonxe2x88x921*meterxe2x88x922 (mNxe2x88x921 mxe2x88x922) and is calculated using the following formula   B  =      1          3      ·      E      ·      I      
wherein xe2x80x9cExe2x80x9d represents the Young""s modulus of elasticity (Giga Pascal) of the fiber; and xe2x80x9cIxe2x80x9d represents the moment of inertia (mm4) of the individual fiber body. A fiber having a lower bendability xe2x80x9cBxe2x80x9d will of course be less flexible than a fiber having a higher bendability xe2x80x9cB.xe2x80x9d The moment of inertia xe2x80x9cIxe2x80x9d describes the property of matter to resist any change in movement or rotation. For a cross-sectional profile having a generally quadrilateral (or approximately rectangular) shape, the moment of inertia can be calculated using the formula
Irectangle=1/12xc2x7w xc2x7t3 
wherein xe2x80x9cwxe2x80x9d represents the average width of the rectangle and xe2x80x9ctxe2x80x9d represents the average thickness of the rectangle.
In further exemplary embodiments, the xe2x80x9cbendabilityxe2x80x9d of fibers can be further improved if the thickness and/or the width of the fibers are varied along the length of the fibers, for example from 2.5-25 percent maximum deviation from the average thickness or width value. This small variation of the thickness and/or the width of the fiber also improves the bond between the reinforcing matrix and the fiber.
The inventors realized, in view of the above equation for xe2x80x9cbendabilityxe2x80x9d xe2x80x9cBxe2x80x9d of fibers having generally quadrilateral cross-sections, that an increase in the fiber modulus of elasticity xe2x80x9cExe2x80x9d will result in a corresponding decrease in bendability and, consequently, make fiber dispersibility more difficult. The inventors then realized that to maintain the same level of bendability, the moment of inertia xe2x80x9cIxe2x80x9d must be decreased, and this could be achieved, for example, by reducing the thickness of the fibers while maintaining the cross-sectional area of the fibers.
In further embodiments of the invention, preferred individual fiber bodies have the following properties when measured in the longitudinal dimension (end to end) along the axis of the fiber body: a Young""s modulus of elasticity of 3-20 Giga Pascals and more preferable 5-15 Giga Pascals, a tensile strength of 350-1200 Mega Pascals and more preferable 400-900 Mega Pascals, and a minimum load carrying capacity in tension mode of 40-900 Newtons more preferable 100-300 Newtons.
A particularly preferred method for manufacturing the fibers is to melt-extrude the polymeric material (e.g., polypropylene as a continuous sheet); to decrease the temperature of this extruded sheet melt below ambient temperature (e.g., below 25xc2x0 C.); to cut or slit the sheet (after cooling) into separate or separable individual fiber bodies having generally quadrilateral cross-sections to stretch the individual fibers by at least a factor of 10-20 and more preferably between 12-16, thereby to achieve an average width of 1.0-5.0 mm and more preferably 1.3-2.5 mm and an average thickness of 0.1-0.3 mm and more preferably 0.15-0.25 mm; and to cut the fibers to obtain individual fiber bodies having an average fiber length of 20-100 mm and more preferably between 30-60 mm. Further exemplary processes are described hereinafter.
The present invention is also directed to matrix materials, such as concrete, mortar, shotcrete, asphalt, and other materials containing the above-described fibers, as well as to methods for modifying matrix materials by incorporating the fibers into the matrix materials.
Further advantages and features of the invention are further described in detail hereinafter.